The present invention relates to magnetic particle powder of acicular ferric oxide used for a magnetic recording material and a process for producing the same. The present invention relates particularly to magnetic particle powder of acicular ferric oxide suitable for a magnetic recording material of, for example, a magnetic tape or a magnetic disk used in magnetic recording such as audio recording, video recording or the like, and a process for producing the same.
In recent days, recording signals increasingly tends to be transferred into the short wave length range, thereby demanding an improvement of a high frequency characteristic of a magnetic recording material and thus particularly demanding an intensive coercive force of magnetic particle powder of ferric oxide used as a magnetic recording material.
As conventionally known, a typical process for preparing magnetic particle powder comprises, for example as disclosed in Japanese patent publication No. 7776/1951, reducing acicular goethite particle powder as the starting material to obtain magnetite particle powder, oxidizing said magnetite particle powder and thereby obtaining gamma-hematite (maghemite) particle powder retaining the acicular shape of the starting material.
Further, the following three kinds of ordinary methods for improving the coercive force of gamma-hematite (maghemite) particle powder have been proposed.
(A) A method for increasing the coercive force comprising increasing the shape anisotropic energy of magnetic particle powder by increasing the axis ratio (long axis/short axis) of each acicular magnetic particle.
(B) A method for increasing the coercive force comprising increasing the crystal anisotropic energy of each magnetic particle by adding metal other than iron e.g. nickel thereto.
(C) A method for increasing the coercive force comprising making close the crystal organization of each acicular magnetic particle.
The present invention relates to the abovementioned ordinary method (C), which is based on the following view.
In the conventional method comprising reducing acicular goethite particle powder as the starting material to obtain magnetite particle powder and then oxidizing said magnetite particle powder and thereby obtaining acicular gamma-hematite (maghemite) particle powder, it is unavoidable that holes are formed inside the particles during the reducing and oxidizing steps as has been already reported.
Therefore, the crystal organization of produced particles becomes rough, which results in hindering the improvement of magnetic characteristics, especially of the coercive force.
From this reason, various kinds of proposals have been made from the standpoint that making close of the crystal organization of magnetic particles can improve the magnetic characteristics thereof. The conventional methods belonging to the abovementioned method (C) are, for example, as follows.
A method comprising reducing magnetic ferric oxide at a low temperature to obtain Fe.sub.3 O.sub.4 and then annealing the same at a temperature of 800.degree. - 1000.degree. C. under such a condition as does not change the composition of Fe.sub.3 O.sub.4.
Japanese patent publication No. 26156/1963
A method comprising reducing acicular ferric oxide at a temperature of 300.degree. - 500.degree. C. in reducing gas (to the state FeOx; X&lt;1) and then oxidizing the same in an oxidizing atomosphere to FeOx with X&gt;1.3.
Japanese patent publication No. 5009/1964
However, according to the abovementioned conventional methods it is difficult to produce on an industrial scale magnetic particle powder of acicular ferric oxide having a high coercive force as required today to be used for a magnetic recording material.
Further, as well-known, magnetic particle powder of acicular ferric oxide used for a magnetic recording material e.g. for a magnetic tape, a magnetic disk or the like requires not only excellent magnetic characteristics but also powder characteristics e.g. an excellent orientation within a coating film and a sufficient evenness on the coating surface in case of being formed into a magnetic coating using resins as a binder. However, according to conventionally proposed processes for producing magnetic particle powder of ferric oxide it has been difficult to obtain magnetic particle powder of ferric oxide having sufficient powder characteristics.
An object of the present invention is to provide magnetic particle powder of acicular ferric oxide having a high coercive force suitable for a magnetic recording material, and a process for producing the same.
Another object of the present invention is to provide an industrially simple and new process for producing said magnetic particle powder of ferric oxide having excellent powder characteristics.
The abovementioned objects of the present invention are achieved by a process of the present invention as summarized below.
A process for producing magnetic particle powder of acicular ferric oxide used for a magnetic recording material according to the present invention comprises
(A) reducing acicular goethite particle powder to obtain acicular magnetite particle powder;
(B) heating said acicular magnetite particle powder at 200.degree. - 800.degree. C. in a heating atmosphere in which the partial pressure of oxygen is controlled at 0.001- 1% by weight and thus making the partial pressure of oxygen of said magnetite particle powder reach the equilibrium value under said condition; and
(C) oxidizing said heat-treated acicular magnetite particle powder to obtain gamma-hematite (maghemite) particle powder retaining the acicular shape of the starting material.
Another process for producing magnetic particle powder of ferric oxide according to the present invention comprises the abovementioned process wherein said acicular goethite particle powder as the starting material includes 0.01- 1.0 % by atom of Cr component with respect to Fe component contained therein.